The present invention relates to an image display device, and is particularly suitable to a high-resolution liquid crystal display device in which disorder of the initial orientation of liquid crystal is reduced to improve image quality, and an image display device using a thin substrate (flexible display), for example, using a plastic substrate. The invention also relates to a method for manufacturing the image display device, and particularly to a method for manufacturing the image display device using a transfer method.
As image display devices of televisions and information terminals, a liquid crystal display device in which liquid crystal material is sealed between a pair of substrates is widely used. A liquid crystal display device of this kind typically includes a plurality of scan signal lines and a plurality of image signal lines, which cross the scan signal lines and are insulated therefrom, on the principal surface (inner surface) of one of the pair of substrates. A pixel is formed in the area surrounded by two adjacent scan signal lines and two adjacent image signal lines. For each pixel, a switching element for controlling on and off, a pixel electrode and the like are formed.
In general, a thin film transistor (TFT element) is used as the switching element. Therefore, one of the substrates on which thin film transistors are provided is typically referred to as a thin film transistor substrate (TFT substrate), and a panel including the TFT substrate (one panel) is also referred to as a rear panel. The other panel, which is paired with the rear panel, includes color filters for a plurality of colors formed on the principal surface of the substrate referred to as a counter substrate in such a way that the color filters correspond to pixels provided on the TFT substrate. The other panel is therefore also referred to as a front panel. The counter substrate is also referred to as a color filter substrate (CF substrate).
Examples of methods for driving a liquid crystal display device are categorized in terms of pixel driving method as a vertical electric field method, such as a TN, method and a VA method, and a horizontal electric field method known as an IPS (In-plane-Switching) method. In the vertical electric field method, the counter electrode (also referred to as a common electrode) that faces the pixel electrode on the TFT substrate that forms the rear panel is provided on the counter substrate that forms the front panel. In the horizontal electric field method (hereinafter referred to as an IPS method), the counter electrode is provided on the TFT substrate on which the pixel electrode is formed.
On the interface between the TFT substrate, which forms the rear panel, and the liquid crystal material, and the interface between the counter substrate, which forms the front panel, and the liquid crystal material, there are provided orientation films having a function (liquid crystal orientation control function) of controlling the direction of the liquid crystal molecules when there is no potential difference between the pixel electrode and the counter electrode (initial orientation) as well as controlling the arrangement and inclination of the liquid crystal molecules when there is a potential difference between the pixel electrode and the counter electrode.
In particular, an IPS liquid crystal display device is known to have a wide viewing angle because the motion of the liquid crystal molecules is rotation in a plane parallel to the substrate surface, so that the optical characteristics do not greatly change with the viewing angle. However, to achieve a large viewing angle, the initial orientation (pretilt) of the liquid crystal molecules needs to be as parallel to the in-plane direction as possible. The orientation film is provided on the TFT elements, wiring lines and the like on the TFT substrate that forms the rear panel, and the orientation film is provided on the black matrix and the color filters on the CF substrate that forms the front panel. By forming the orientation films as flat as possible, the pretilt angle is reduced.
FIG. 8 is a plan view showing an example of arrangement of wiring lines, electrodes, and drive elements for pixels in a conventional IPS liquid crystal display device. FIG. 8 shows only two pixels adjacent in the direction in which gate lines extend. As shown in FIG. 8, gate lines 801 and data lines 802 are periodically disposed on a TFT substrate (not shown) and connected to drive elements (TFT elements) 803. The drive element 803 applies a voltage according to an image signal from the data line 802 to a drive electrode (pixel electrode) 804, so that an electric field induced between the pixel electrode 804 and a counter electrode 805 causes in-plane rotation of the liquid crystal molecules and hence off-to-on transition of the image (pixel). In FIG. 8, let L be the pixel size in the direction in which the data lines 802 are disposed.
FIGS. 9A and 9B are schematic cross-sectional views taken along the direction in which the data lines for the conventional IPS pixels described with reference to FIG. 8 are disposed. Drive elements 911, drive electrodes 912 and counter electrodes 913 are disposed on a rear panel 910, which is one panel, and color filters 921, 922 and 923, for example, having three separate colors (red: R, green: G, and blue: B) and a light blocking layer (black matrix) 924 between each adjacent pair of the color filters are disposed on a front panel 920, which is the other panel. A liquid crystal layer 930 is sandwiched between the rear panel 910 and the front panel 920.
FIG. 9A shows an exemplary case where the pixel size is L1, and FIG. 9B shows an exemplary case where the pixel size is reduced to L2 for higher resolution. In the rear panel 910, the sizes of the drive element 911, the drive electrode 912, and the counter electrode 913 cannot be easily reduced because of resolution constraint of photolithography used in the manufacturing process. Furthermore, reduction in film thickness of various functional films formed on the substrate surface is limited from the viewpoint of stable operation of the drive element and the like. Thus, the surface roughness of the interface between the rear panel 910 and the liquid crystal layer 930 increases with the size reduction from L1 to L2.
Similarly, reduction in thickness of the light blocking layer 924 and the color filters 921, 922 and 923 on the front panel 920 is limited from the viewpoint of optical characteristics. Thus, the surface roughness increases in the vicinity where the light blocking layer 924 is in contact with the color filters 921, 922 and 923.
Since the IPS method uses liquid crystal molecule rotation in the plane parallel to the substrate surface to control on and off of the pixels, it is important that the liquid crystal molecules are oriented parallel to the substrate surface. The so-called pretilt angle is desirably smaller than or equal to 2 degrees at the maximum, more desirably smaller than or equal to 1 degree if possible. Therefore, the interfaces where the rear panel 910 and the front panel 920 come into contact with the liquid crystal layer 930 are required to be as flat as possible. However, when the pixel size is reduced for higher resolution, the surface roughness of the interface increases from the reason described above. This causes disorder of the orientation of the liquid crystal molecules, resulting in contrast reduction and color shift.
JP-A-10-288796 describes a structure in which a color filter layer is provided on a rear substrate (TFT substrate) side and a drive element (TFT element) is formed on the color filter layer. In this structure, an insulating film for planarization is provided on the color filter layer including color filters and a black matrix formed on the TFT substrate. However, JP-A-10-288796 relates to a solution to display unevenness due to impurity ions from the color filter layer, but does not aim to reduce disorder of the initial orientation, which the invention is to solve. JP-A-2002-184959 relates to a transfer method similar to that used in the invention but does not consider at all the configuration of the TFT element, color filters and the like.